1. Field of the Invention
The present invention relates to universal series-excitation and compound excitation electric motors and, more particularly, to a speed limiting circuit for controlling the speed of such motors under load and no load condition.
2. Description of the Background
Conventional universal series or compound electric motors are widely utilized in driving various kinds of industrial machinery, electric tools, and household appliances. However, these types of motors have an undesirable characteristic in that they are very load sensitive.
Series motors develop a high torque at starting and run at a speed which is dependent on load. As the load on the motor is decreased, the speed of the motor will increase. The load dependency of series and compound motors is due to the method in which the field and armature windings are connected. In the series motor, the field and armature windings are connected in series so that the strength of the field is dependent on the motor load and varies with armature current. The load dependency is a desirable characteristic in many applications, including industrial machinery which must initially accelerate at high torque to a higher continuous speed and lower operating torque. However, under no load conditions, the high rotational speed causes noise and wear on the gear and bearings of the transmission device. Under high load conditions, the rotational speed slows, the field winding becomes saturated, and efficiency is decreased.
The speed/load dependency is reduced in conventional shunt motors where the field winding is connected in parallel with the armature. This way, the field winding is energized by a constant voltage so that the magnetic field remains constant. Consequently, the speed remains relatively constant under varying load conditions. However, shunt motors lack the inherent high starting torque of series-excitation motors.
A compound motor is a hybrid of the above-described series and shunt motors. In addition to the shunt field winding in parallel with the armature, the compound motor has a series winding which reinforces the field. Compound motors have a high starting torque similar to series motors and exhibit some degree of speed constancy as in the shunt motor. Nevertheless, the variance in speed of compound motors can have detrimental effects similar to those of series motors, such as wear on the transmission device and high speed noise.
Hence, there exists a clear commercial need for a device capable of limiting the maximum and/or minimum speed of series motors, and of maintaining tighter control of the maximum and/or minimum speed of compound motors.